Solar - Series Or Parallel?

[Dougal]

I know there's a big thread on solar, but it's not mine, and asks a different question...

I'm installing 2 x 50watt panels and a Victron 75/15 MPPT controller (75volt max & 15amp). We're aboard the boat all summer in southern Brittany, so a fair amount of sun. The boat normally stays afloat all year, the only electrics left on, being the bilge pump and sensor (not a constant draw item).

Would you fit in parallel or series?

 

[PaulRainbow]

I know there's a big thread on solar, but it's not mine, and asks a different question...

I'm installing 2 x 50watt panels and a Victron 75/15 MPPT controller (75volt max & 15amp). We're aboard the boat all summer in southern Brittany, so a fair amount of sun. The boat normally stays afloat all year, the only electrics left on, being the bilge pump and sensor (not a constant draw item).

Would you fit in parallel or series?

Series.

 

[overstag]

If there’s little chance of shading of either panel, go series. I you’re like
Y to get some shading of the panels, go paralel.

 

[GHA]

If there’s little chance of shading of either panel, go series. I you’re like
Y to get some shading of the panels, go paralel.

:encouragement:

Just a small area on one panel will hammer the current out of both panels if in series.

https://www.youtube.com/watch?v=ofo1HQyGG8s

 

[PaulRainbow]

:encouragement:

Just a small area on one panel will hammer the current out of both panels if in series.

https://www.youtube.com/watch?v=ofo1HQyGG8s

So, looking at the video you linked to, you would advise the OP to fit his panels in parallel, if there could be some shading, even though the PV is only 16.5v and the controller output is as low as 12.57v ?

 

[Poecheng]

If there’s little chance of shading of either panel, go series. I you’re like
Y to get some shading of the panels, go paralel.

Isn't this the wrong way round? Series will will create 36v from 2 x 18v panels whereas parallel will create 18v but a greater current. In shady conditions, you want to keep the voltage up (series) whereas in sunny conditions you have sufficient voltage and want maximum current.
With a Victron MPPT controller, I can see no good reason for not having the panels in series as long as the overall voltage is not in excess of the controller's range. It means you can always get sufficient voltage for the controller to work (even if shade) and, if there is no shade and therefore ample voltage, then it converts the 'extra' voltage to current.

 

[Tim Good]

I choose to wire my 360w array of 3 panels in series into a Victron MPPT. Reason is because I have a big boat and the cable run from the panels to the batteries is a long way and I needed to keep the amps down to avoid running an excessively large cable.

However, there is no doubt that partial shading will effect them more. Mine are set above the cockpit and I ensure nothing is above them like antennas etc... so for me series works. If I could be arsed to run a fat cable then I'd maybe opt for parallel.

My victron regularly tells me I can reach over 360w on a good day and I can generate 2kw if necessary so series must be working ok!

 

[sailaboutvic]

Just to throw another question here .
We have two 150 panels wired parallel with a MPPT .
We plainning to add 2 x 100 panels which we going to need another MPPT controller would it be ok to wired the new panels in series ? If not why not ?
Also would there be a problem having two MPPT controllers as in one would over ride the other one ?

 

[lw395]

Just to throw another question here .
We have two 150 panels wired parallel with a MPPT .
We plainning to add 2 x 100 panels which we going to need another MPPT controller would it be ok to wired the new panels in series ? If not why not ?
Also would there be a problem having two MPPT controllers as in one would over ride the other one ?

Having separate MPPT control over the two sets of panels is not a problem.
Having the two sets wired differently will be fine, provided the two systems are only connected at the batteries.
You could have issues with the 'smart charger' side of the controllers though, as one charger may sense the battery bank as fully charged, due to the other charger. It may be possible to link the control of the two chargers, maybe in a master/slave context. RTFM I'm afraid.

 

[Tim Good]

As LW395 said above... not a problem to have a seperste array. However some have suggested the two controllers could conflict. As such some have suggested to program one tonhave a slightly different voltage setting so one is primary.

That said I didn't find it an issue and didn't have to alter anything. I have an MPPT and a standard controller working side by side and they don't appear to affect one another.

In my case I have 360w in series into an MPPT and another panel that roams on a long cable.

 

[PaulRainbow]

So, looking at the video you linked to, you would advise the OP to fit his panels in parallel, if there could be some shading, even though the PV is only 16.5v and the controller output is as low as 12.57v ?

As we don't have an answer to this question, just another example of unsubstantiated advice, i'll answer it myself.

Given the data from the video GHA linked to, connecting those two panels in series to the Victron MPPT controller would give you absolutely zero output from your solar panels.

Why ? The Victron controller will not start unless the panel voltage exceeds the battery voltage by 5 volts. So with just 16.5 volts at the panel you'd need the batteries to be 11.5 volts, as in flat.

 

[noelex]

It would be great to have more test results on the practical difference in output between series or parallel connection, but the overwhelming evidence is that parallel connection will give more overall output if the panels are mounted on a boat.

There are a couple of caveats. The Victron controllers have an unusually high start up voltage criterion. This seems to cause little practical problems, but if your panels have a relatively low Voc, you live in a hot climate, or there is another source of charging used at night (such as wind generator) it is possible the start up criterion may not be met early in the solar day if the panels are wired in parallel. This may in some cases swing the advantage back to series connection with the Victron controllers.

My advice would be wire the panels in parallel , but check that the start up current is low.

 

[noelex]

Multiple controllers are generally not a problem, in fact the modern way to wire solar panels for maximum efficiency is one controller per panel. If the panels are a reasonable size this will usually produce more output than series or parallel connection.

Once again, there are some caveats. The Victron controllers have a feature where if the absorption set point is met and the current drops below a threshold (the threshold depends on the model, but is around 1A) the controller will drop to float. This feature can be very frustrating when multiple controllers are used. I am hoping Victron will release a software update to disable this function in the same way the temperature compensation can be switched off.

 

[PaulRainbow]

Just to throw another question here .
We have two 150 panels wired parallel with a MPPT .
We plainning to add 2 x 100 panels which we going to need another MPPT controller would it be ok to wired the new panels in series ? If not why not ?
Also would there be a problem having two MPPT controllers as in one would over ride the other one ?

It would be ok to connect the new panels in series. There is a danger with two controllers, in that one could, as you say, "override the other". With your proposed arrangement the best chance of avoiding that would be to have two controllers with exactly the same charging profile.

Another, and in my opinion better, solution would be to connect the two existing panels in series, the two new panels in series, then connect the two pairs together in parallel.

 

[lw395]

Multiple controllers are generally not a problem, in fact the modern way to wire solar panels for maximum efficiency is one controller per panel. If the panels are a reasonable size this will usually produce more output than series or parallel connection.

Once again, there are some caveats. The Victron controllers have a feature where if the absorption set point is met and the current drops below a threshold (the threshold depends on the model, but is around 1A) the controller will drop to float. This feature can be very frustrating when multiple controllers are used. I am hoping Victron will release a software update to disable this function in the same way the temperature compensation can be switched off.

You have to have some sort of strategy to switching to float mode (and back again), unless you want your batteries to be frazzled by permanent boost charging. That's why we have these smart chargers in the first place.
The best answer is probably to separate the functions of extracting power from the panels and controlling charging of the batteries.

 

[noelex]

As we don't have an answer to this question, just another example of unsubstantiated advice, i'll answer it myself.

Given the data from the video GHA linked to, connecting those two panels in series to the Victron MPPT controller would give you absolutely zero output from your solar panels.

Why ? The Victron controller will not start unless the panel voltage exceeds the battery voltage by 5 volts. So with just 16.5 volts at the panel you'd need the batteries to be 11.5 volts, as in flat.

It has been some time since I watched the video. Unfortunately, my internet connection is not sufficient to refresh my memory, but I think you are confusing Voc and Vmp.

The Voc has to be +5v for the Victron to start. The 16.5V is a Vmp measurement.

Once started, the criterion drops to +1v.

 

[lw395]

It would be ok to connect the new panels in series. There is a danger with two controllers, in that one could, as you say, "override the other". With your proposed arrangement the best chance of avoiding that would be to have two controllers with exactly the same charging profile.

Another, and in my opinion better, solution would be to connect the two existing panels in series, the two new panels in series, then connect the two pairs together in parallel.

Two chargers with the same set points can still have problems, as a small current from one charger is normally enough to stop the other from leaving float mode.

 

[lw395]

It would be great to have more test results on the practical difference in output between series or parallel connection, but the overwhelming evidence is that parallel connection will give more overall output if the panels are mounted on a boat.

There are a couple of caveats. The Victron controllers have an unusually high start up voltage criterion. This seems to cause little practical problems, but if your panels have a relatively low Voc, you live in a hot climate, or there is another source of charging used at night (such as wind generator) it is possible the start up criterion may not be met early in the solar day if the panels are wired in parallel. This may in some cases swing the advantage back to series connection with the Victron controllers.

My advice would be wire the panels in parallel , but check that the start up current is low.

Or you need a more suitable controller perhaps?

 

[PaulRainbow]

Two chargers with the same set points can still have problems, as a small current from one charger is normally enough to stop the other from leaving float mode.

Indeed, that's why i said "best chance of avoiding that", it can still happen though, as you say. Some people seem to base their advice on snippets of "information" gleaned from the web, based on other types of installation. For instance, a bank of panels on a house roof will all see the same Sun, all of the time. Whereas on a boat panels can be in very different locations. I have a pair on an arch at the transom and another pair on the coachroof, just ahead of the sprayhood. There will be days when the pair on the arch will be in a significant amount of sunshine before the pair on the coachroof are out of the shadow of the sprayhood.

Each pair is wired in series and the two pairs are then in parallel, connected to a single controller. Two smart controllers on a boat isn't a good combination. For instance, my mains charger will stop the solar controller dead in its tracks. I have a solution in place for those times of year when the solar yield is insufficient and i need some extra power, from the mains charger. I can have the mains charger coming on automatically, as and when needed, to keep the batteries at an acceptable SOC, whilst still using as much of the solar yield as possible.

 

[noelex]

You have to have some sort of strategy to switching to float mode (and back again), unless you want your batteries to be frazzled by permanent boost charging. That's why we have these smart chargers in the first place.

Yes, agreed. The Victron feature (they call it tail current cut off) is only a very minor part of the charging algorithm. It is not designed to work as the primary means of determining the drop to float.

Indeed, in most installations it will never be activated. Unfortunately, it does not work with multiple charge sources. If you have multiple charge sources, as most boats do, it is a feature that needs to be disabled for the charge algorithm to work.

There are some “work arounds” for this problem, but they are less than ideal.

The best answer is probably to separate the functions of extracting power from the panels and controlling charging of the batteries.

It would be ideal to separate the MPPT circuitry, install this for each solar panel, but have a global controller of the charge algorithm.